The classical way of calibrating instruments for measuring resistance and/or conductance has been to connect a precision resistor of known value across the input terminals of the measuring instrument, read the instrument and record the difference (e.g., error) between the reading and the known value of the precision resistor. Several such calibration sequences are performed over the range of impedances that can be measured by the measuring instrument; and, the results are used to form a table or graph. Thereafter, when the measuring instrument is used to measure the value of an unknown resistance, the reading is corrected using the calibration error displayed on the table or the graph. Measurement errors lying between the points where calibration errors are determined, of course, require the use of interpolation techniques.
The foregoing method of calibrating resistance and conductance measuring instruments has a number of disadvantages. First, the foregoing sequence is time consuming and, therefore, expensive. Further, it must be carried out by skilled electronic technicians. Also, while precision resistors are relatively accurate when compared with nonprecision resistors, their resistance values are not precisely exact, i.e., their resistance values fall within some (albeit small) tolerance range. Moreover, the resistance values of even precision resistors vary with temperature and age. As a result, the precision resistors used for calibration purposes must themselves be checked for accuracy. Checking the calibrating resistors is, of course, also time consuming and, therefore, costly. Moreover, because of instrument component aging, deterioration and the like, resistance/conductance measuring instruments must be frequently recalibrated in order for their associated error tables or graphs to be relied on to produce accurate results. Regular recalibration is equally time consuming and, therefore, also costly. Finally, the accuracy of error tables or graphs is directly related to the number of error points or values determined during calibration. If a small number of calibration error values are determined, the ranges over which interpolation is required will be relatively wide. Because interpolation is usually performed in a linear manner whereas errors are frequently nonlinear (and often random), the width of the interpolation regions is often directly related to the inaccuracy remaining in "corrected" measurements. Therefore, there is a need for a method and apparatus for rapidly and automatically determining the calibration error of resistance/conductance measuring instruments. It is a general object of this invention to provide such a method and apparatus.
That is, it is an object of this invention to provide a method and apparatus for rapidly and automatically calibrating resistance and/or conductance measuring instruments.
It is another object of this invention to provide a method and apparatus for determining the calibration error of resistance and/or conductance measuring instruments that does not involve the use of highly precise resistors during a time consuming series of calibration sequences.
It is a further object of this invention to provide an apparatus for calibrating resistance and/or conductance type measuring instruments that is rapid and easy to use, particularly by nonhighly skilled personnel.